The surfaces of the solar system’s big, rocky bodies all look a lot alike — they’re rugged and barren. But below the surface, they can be quite different.

Two examples play a game of “peek-a-boo” in the wee hours of tomorrow morning — the Moon and Ceres, the largest asteroid. The Moon will cover up Ceres for about 45 minutes or so, with the exact timing depending on your location. Ceres is quite faint, so you need a telescope to see it before and after it disappears behind the Moon.

The Moon’s surface sits atop layers of rock. But the surface of Ceres appears to sit atop a layer of water — some of it frozen, but perhaps some of it liquid. Christopher Russell is the lead scientist for Dawn, a mission that’s supposed to enter orbit around Ceres in 2015.

RUSSELL: We think there’s subsurface liquid water. We think that, maybe, even some of that water gets out to the surface. That’s one of the things we’ll be looking for is evidence that, perhaps in the equatorial regions, where it’s a little warmer, as to whether water is leaking out on the surface.

Ceres is only about a quarter the size of the Moon, and only about one percent as massive. That means its density is quite low, indicating that it may have a substantial amount of water — a layer perhaps 50 or 60 miles thick. And much of its surface is made of clay, which is formed in a wet environment.

Dawn will help refine that picture of Ceres, telling us what’s beneath the skin of the largest asteroid.